Coal-Fired Power Generation Handbook. James G. Speight

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СКАЧАТЬ cooling system, could not be expected to achieve the same efficiency as one without flue gas desulfurization units using high-rank, low-ash, and low-moisture bituminous coal at a coastal site with cold seawater cooling. In most cases, there is little that can be done to mitigate these effects; it is sufficient to recognize that their impact is not necessarily a result of ineffective design or operation, but merely a function of real plant design constraints.

      The efficiency of converting coal into electricity is of prime importance since more efficient power plants use less fuel and emit less climate-damaging carbon dioxide. However, with many different methods used to express efficiency and performance, it is often difficult to compare one coal-fired plant with another, even before accounting for any fixed constraints such as coal quality and cooling-water temperature. Guidelines are required that allow the efficiency and emissions of any plant to be reported on a common basis and compared against best practice. Such comparisons start with the classification of coal and, amongst other parameters, allow less efficient plants to be identified and steps taken to improve these plants. Having such information available will allow better monitoring of plant performance and, if necessary, regulate the means by which coal is used for power generation, leading to a more sustainable use of coal.

2.2 Nomenclature of Coal

Coal is classified into three major types, namely anthracite, bituminous, and lignite. However, there is no clear demarcation between them and coal is also further classified into subcategories as semi-anthracite, semi-bituminous, and subbituminous. Anthracite is the oldest coal from geological perspective and it is a hard coal composed mainly of carbon with little volatile content and practically no moisture. Lignite is the youngest coal from geological perspective and it is a soft coal composed mainly of volatile matter and moisture content with low fixed carbon. Fixed carbon refers to carbon in its free state, not combined with other elements. Volatile matter refers to those combustible constituents of coal that vaporize when coal is heated (Speight, 2013).

      The different types of coal contain both organic and inorganic phases. The latter consist either of minerals such as quartz (SiO₂) and various clay minerals that may have been brought in by flowing water or by wind activity or minerals (such as pyrite, FeS2, and marcasite) that are formed in place (authigenic minerals). The minerals can have a major effect on the efficient use of coal and should be removed before use. Other properties, such as hardness, grindability, ash-fusion temperature, and free-swelling index (a visual measurement of the amount of swelling that occurs when a coal sample is heated in a covered crucible), may affect coal use (especially when coal is used for power generation). Hardness and grindability determine the kinds of equipment used for reducing the size of the coal that enters the combustor (or the gasification unit) and the ash-fusion temperature influences the design of the furnace as well as the operating parameters of the furnace. The free- swelling index provides preliminary information concerning the suitability of a coal combustion and gives an indication of the potential of the coal for coke production, which is another indication of the suitability of the coal for combustion that leads to power generation.

      Because of all of these varying properties, the nomenclature of coal – as might be expected – is not straightforward and requires considerable thought to elucidate the precise meaning of some of the terminology (Chapter 1). However, since coal and coal products will play an increasingly important role in fulfilling the energy needs of society it is essential that coal types be understood before use. In fact, future applications will extend far beyond the present major uses for power generation and chemicals production (Speight, 2013, 2020). A key feature in these extensions will be the development of means to provide analytical data that will help in understanding the conversion of coal from its native form into useful gases, liquids, and solids in ways that are energy efficient, nonpolluting, and economical.

      The design of a new generation of conversion processes will require the analyst to have a deeper understanding of the intrinsic properties of coal and the ways in which coal is chemically transformed to produce energy under process conditions. Coal properties – such as the chemical form of the organic material, the types and distribution of organics, the nature of the pore structure, and the mechanical properties must be determined for coals of different ranks (or degrees of coalification) in order to use each coal type most effectively.

      First and foremost, coal is a sedimentary rock of biochemical origin and is formed from the accumulations of organic matter which occurred along the edges of shallow seas and lakes or rivers. Flat swampy areas that are episodically flooded are the best candidates for coal formation. During non-flooding periods of time, thick accumulations of dead plant material pile up. As the water levels rise, the organic debris is covered by water, sand, and soil. The water (often salty), sand and soils can prevent the decay and transport of the organic debris. If left alone, the buried organic debris begins to go through the coal series as more and more sand and silt accumulates above it. The compressed and/or heated organic debris begins driving off volatiles, leaving primarily carbon behind.

In addition to recognizing coal as an organic combustible sedimentary rock, another part of the challenge is to identify the chemical pathways followed during the thermal conversion of coal to liquids or gases (Speight, 2013, 2020). This is accomplished by tracing the conversion of specific chemical functional groups in the coal and studying the effects of various inorganic compounds on the conversion process. Significant progress has been made in this area by combining test reactions with a battery of characterization techniques. The ultimate goal is to relate the structure of the native coal to the resulting conversion products.

      There is also a major challenge to the coal analyst and this involves recognizing the heterogeneity of coal – even during the formation of one coal seam, conditions vary and, hence, the types of coal vary depending upon the character of the original peat swamp (Speight, 2013, 2020). Within a swamp some areas might be shallow and other areas deep. Some areas might have woody plants and other areas grassy. The environment might be changing over time, making the bottom (the older part) of the coal seam different to the top (the younger part) of the seam. Varying water level and movement changes the degree of aeration and hence the activity of aerobic bacteria in bringing about decay. The different types of chemical substance present in plants (such as cellulose, lignin, resins, waxes, and tannins) are present in different relative proportions in living woody tissue, in dead cortical tissue as well as in seed and leaf coatings, In addition, these substances show differing degrees of resistance to decay.

      Thus, as conditions fluctuate during the accumulation of plant debris, the botanical nature and chemical composition of the material surviving complete breakdown will fluctuate also, not only on a regional basis but also on a local basis. This fluctuation is the origin of the familiar banded structure of coal seams, which is visible to the naked eye, and provides strong support case for the different chemical and physical behavior of coals.

      Furthermore, coal seams, sandstone, shale, and limestone are often found together in sequences hundreds of feet thick. The key to large productive coal beds or seams seems to be long periods of time of organic accumulation over a large flat region, followed by a rapid inundation of sand or soil, and with this sequence repeating as often as possible. Such events happened during the Carboniferous Period – recognized in the United States as the Mississippian and Pennsylvanian time periods due to the significant sequences of these rocks found in several states; other coal-forming periods are the Cretaceous, Triassic, and Jurassic Periods (Chapter 1).

To complicate matters even further, coal is also considered (perhaps without sufficient scientific foundation) to be a metamorphic rock – the result of heat and pressure on organic sediments such as peat – but most sedimentary rocks undergo some heat and pressure and the association of coal with typical sedimentary rocks and its mode of formation usually keep low-grade coal in the sedimentary classification system. On the other hand, anthracite undergoes more heat and pressure and is associated with low grade metamorphic СКАЧАТЬ